In oscillator circuits it is usually necessary to have a feedback element to stabilize the oscillator. An example of such a feedback element is a surface acoustic wave (SAW) resonator. SAW resonators are well known and fully described in the prior art. For example, they are disclosed by Hartmann, et al. in U.S. Pat. No. 3,886,504 and by Shreve in U.S. Pat. No. 4,144,507, both of which are incorporated by reference herein. In these cited references, there is disclosed a SAW resonator employing two spaced grating structures on a piezoelectric substrate, wherein at least one surface acoustic wave transducer is interposed between the grating structures for generating surface acoustic waves in the piezoelectric material in response to electrical excitation and with the grating structures being so positioned in relation to each other to set up a standing wave resonance condition from the reflection of surface acoustic waves generated by the transducer. The resonance condition of the SAW resonator, then, can be used to stabilize an oscillator circuit.
In order to optimize the stabilization and maximize the signal to noise ratio of an oscillator, it is necessary to maximize the power handling capability in the frequency stabilizing feedback element while maintaining other desirable properties such as high Q and low insertion loss. In a SAW resonator, the power handling ability is proportional to the acoustic aperture or width of the device. Thus, it is desirable to have large acoustic apertures in SAW resonators used to stabilize oscillators. However, a limitation on aperture size arises. Because the SAW resonator structure is essentially a two-dimensional waveguide, the resonator can support a family of transverse waveguide modes whose frequency separations are inversely proportional to the square of the aperture dimension. This phenomenon is fully described by Hermann Haus, "Modes in SAW Grating Resonators", J. Appl. Phys. 48 (12), December 1977, pp. 4955-61. As a result, for wide apertures which can handle high power, some transverse modes occur at frequencies very close to the fundamental frequency and can cause substantial distortion of the SAW resonator filter response. This distortion can be manifested, for example, as a reduced Q of the resonator. It can also increase phase noise when the resonator is incorporated into an oscillator and thereby cancel the advantages gained by using a wide aperture. For these reasons, whenever a SAW resonator is used to stabilize an oscillator, it is highly desirable to suppress these transverse modes in the SAW resonator.
In the prior art, various methods of suppressing spurious modes have been attempted. One of the most widely reported methods of suppressing spurious modes is the method of apodization. in this technique, the transducer design is tailored to the fundamental mode of the reflector grating structure such that the transverse modes are not excited. As the frequency of the SAW resonator is increased above about 500 MHz, however, the suppression of transverse modes by this method is no longer a simple matter. A design can be made to suppress transverse modes for specific device parameters, but slight variations that are commonly encountered in resonator fabrication in either transducer metalization or grating definition result in a re-emergence of the problems associated with transverse modes. As a result, it has been reported by workers in this field, e.g., Tanski and Cross, et al. in the Ultrasonics Symposium Proceedings, 1979 and 1980, respectively, that there exists a maximum allowable aperture for SAW resonators without transverse mode distortion.
A second method used in the prior art in suppressing transverse modes is to use absorbers to attenuate the transverse modes. The absorber, however, inherently degrades the Q of the resonator since both the desired mode and undesired mode must pass through the absorber.
The spurious frequencies caused by transverse modes are determined by the width of the SAW resonator. In a narrow resonator, the undesired transverse modes are farther from the main modes than in the case of a wide resonator. But narrow resonators have a low power handling capability. To overcome this disadvantage, the resonator in accordance with the preferred embodiment of the present invention provides several individual SAW sub-devices operating in parallel. This parallel combination of SAW sub-devices enables the resonator to handle higher power.
Because of many limitations thought to be associated with it, this parallel technique apparently has not been used for suppressing or minimizing transverse modes. For example, this embodiment of a SAW resonator, that is, connecting sub-devices in parallel to form a single device, has not been used in the past because the resonator structure itself is more complicated, thus requiring more complicated connector structures to make electrical contacts to the parallel resonators.
Still another limitation is the possible introduction of spurious responses by slight variations among the sub-devices. These spurious modes present problems, such as reduced Q of the device and increased oscillator phase noise, that are similar to those generated by transverse modes. In the preferred embodiment, this limitation is overcome by making identical sub-devices on a common substrate.
Another limitation of this technique of parallel operation of devices is the added difficulty in making connections between the parallel beams comprising the sub-devices of this embodiment. In the usual structure of a SAW resonator, connections are made by wide bars that are deposited next to the transducers. In a parallel configuration, these bars must cross through the individual sub-devices. The bars, themselves, can cause spurious resonances and a concomitant degradation of the resonator filter response. This limitation is overcome with the preferred embodiment in accordance with the present invention.
Still another limitation thought to exist with the parallel structure is the added diffraction loss associated with narrow acoustic beams in parallel. Although this loss exists, it has not been found to have been aggravated by a parallel connection of SAW sub-devices because this embodiment is most useful at frequencies where other losses are dominant.
Hartmann, et al., in the reference cited above, made a passing suggestion of paralleling the operation of individual SAW devices for a multi-resonator structure. Their purpose, apparently, was to achieve a desired overall frequency response comparable to that from a multi-pole bandpass filter. No disclosure as to how to practically achieve a parallel operating SAW device or to minimize transverse modes therewith was made. Nor was there a teaching of a parallel operation of individual SAW devices for reducing or suppressing transverse mode distortions.